1. Field of the Invention
The invention relates to a method for producing a sintered rare-earth magnet that has a polycrystalline phase with nanosized crystal grains, a sintered rare-earth magnet, and a material for the same.
2. Description of the Related Art
Rare earth magnets typified by neodymium magnets (Nd2Fe14B) have a high flux density and are used in a variety of applications as extremely powerful permanent magnets. To obtain even better magnetic properties a rare earth magnet with nanosized crystal grains is required.
The following process is available as a typical process for producing a rare-earth magnet by sintering:                1) An alloy melt that has the composition of the rare earth magnet is rapidly solidified and formed into a ribbon (rapidly quenched ribbon) using a single-roll process, double-roll process, etc.; and        2) The ribbon is pulverized into a powder, and the powder is made into a bulk unit by a sintering process such as pressure sintering.        
During rapid solidification in the process of 1) above, however, if an amorphous phase is produced in the rapidly quenched ribbon that is formed thereby, sintering in the process of 2) above also serving as a heat treatment at 600° C. or higher must be performed to crystallize the amorphous phase, and that process causes coarsening of the nanostructure.
Japanese Patent Application Publication No. 09-139306 (JP-A-09-139306) discloses performing sintering by a hot press at the high temperature of 800° C. using powder obtained by pulverizing a rapidly quenched ribbon that has been prepared by the single-roll process as a process for producing a sintered rare-earth magnet. In general, an amorphous phase is produced when rapid quenching is carried out, so the aforementioned kind of high-temperature sintering is carried out to crystallize the entire bulk unit. Coarsening of the crystal grain size is expected as a result.
To enable low-temperature sintering, one solution is to coat the rapidly quenched ribbon with a low-melting point phase with a melting point lower than the ribbon body. Since a low-temperature phase is present on the surface of a powder obtained by pulverizing the rapidly quenched ribbon, a liquid phase will be present on the surface of the powder particles at the time of sintering, and this will enable sintering at a lower temperature.
Japanese Patent No. 02693601 discloses performing rapid quenching by a double-roll process. But with the double-roll process the low melting point phase will formed in the interior of the rapidly quenched ribbon rather than on the surface thereof.
Japanese Patent Application Publication No. 2007-288020 (JP-A-2007-288020) and Japanese Patent Application Publication No. 2007-288021 (JP-A-2007-288021), respectively, disclose coating a rare-earth magnet with Dy (dysprosium) using electrodeposition, or dissolving dysprosium chloride in an organic solvent and using the same. With these methods the dysprosium deposition layer has a thickness of several micrometers and is as thick as the rapidly quenched ribbon. These methods are effective for producing a rare-earth magnet containing dysprosium though internal diffusion of the dysprosium, but these processes cannot produce the thickness of several nanometers that is suitable as a low-melting point phase for rare-earth magnets in general with different compositions. In other words, to form an electrodeposition layer with submicron or even nanosized thickness, it is necessary to perform electrodeposition at a low current and a low metal ion concentration, but rare-earth magnets typified by Nd (Neodymium) and dysprosium have a low reduction potential, so they dissolve in water that is dissolved in the solvent, and it is impossible to perform electrodeposition thereby.
JP-A-2007-288021 also discloses using an ionic solvent as a molten salt, but in this technique it is possible that the crystal grains will be coarsened by the heat of the molten salt, so forming a low temperature phase thereby is essentially impossible.
Japanese Patent No. 02779830 discloses using an anhydrous organic solvent as a surface plating solution for a rare-earth magnet, but it does not disclose information contributing to electrodeposition of a low temperature phase.
In summary, none of the above documents discloses or suggests information that will enable low-temperature sintering to achieve nanosized crystal grains.